Biological components such as the blood and body fat of humans and the like have absorption bands in the near-infrared region, and thus near-infrared spectroscopy has attracted attention as a noninvasive analytical method, and studies and practical applications thereof have been intensively implemented. In particular, recently, diabetes, obesity etc. have been focused on, and absorption spectrum bands of glucose, which is the main component of blood sugar, cholesterol, lipids, and the like lie in the near-infrared region. Thus, studies using the skin of a biological body or the like have been actively performed. In analysis by near-infrared spectroscopy, an output signal includes necessary information and a large amount of noise due to a light-receiving element. Consequently, in order to extract necessary information regarding an output signal without totally depending on an improvement of the performance of sensors (light-receiving elements), a spectroscopic method, chemometrics, or the like has been used as an important method.
In the near-infrared region, the above-mentioned sensors (light-receiving elements) are broadly divided into electron tubes and photodiodes (PDs) which are solid-state components. Among these sensors, PDs have a small size and can be easily highly integrated to form a one-dimensional array, a two-dimensional array, or the like, and thus research and development of PDs has been widely performed (Non-Patent Document 1). The present invention targets a detection device for biological components, the detection device including a PD. Currently, the following PDs or PD arrays are used.
(1) An example such PDs or PD arrays is PDs or arrays thereof having sensitivity up to the infrared region and also having sensitivity in the near-infrared region. Specific examples of such PDs include germanium (Ge)-based PDs, lead sulfide (PbS)-based PDs, HgCdTe-based PDs, one-dimensional arrays thereof, and two-dimensional arrays thereof.
(2) Another example of such PDs or PD arrays is InP-based PDs having sensitivity at a wavelength of 1.7 μm or less in the near-infrared region, InGaAs-based PDs included in the category of the InP-based PDs, and arrays thereof. Herein, the InP-based PDs refer to PDs including an absorption layer composed of a group III-V compound semiconductor and provided on an InP substrate, and InGaAs-based PDs are also included in the InP-based PDs.
Among the above photodiodes, photodiodes described in (1) are often cooled in order to reduce noise. For example, most of the photodiodes are operated under cooling at the liquid nitrogen temperature (77 K) or under cooling with a Peltier device. Accordingly, devices including such photodiodes have a large size, and the device cost is increased. Although such devices can be used at room temperature, the devices have a problem that a dark current is large in the wavelength range of 2.5 μm or less and the detection capability is poor. On the other hand, the InP-base PDs described in (2) have the following disadvantages: (I) In InGaAs, which is lattice-matched to InP, although a dark current is low, the sensitivity of the PD is limited to a wavelength range of 1.7 μm or less in the near-infrared region. (II) In extended-InGaAs, in which the wavelength region where light can be received is extended to 2.6 μm, the dark current is large, and cooling is necessary. Accordingly, in the InP-based PDs, light having a wavelength of 2.0 μm or more, which is important in examinations of biological components, cannot be used or it is necessary to cool the PDs in order to use the light.
In biological component detection using near-infrared light, detection targeting the blood sugar level (such as glucose and grape sugar), which is directly related to diabetes, is most commonly performed (Patent Documents 1 to 4), and next most commonly performed is detection of body fat (Patent Document 5). Furthermore, from the cosmetic standpoint, measurement of collagen related to wrinkles of the skin has been performed using near-infrared light (Patent Document 6). In addition, with regard to a distribution of collagen and the like during a surgery of the cornea, a measurement of infrared rays has been proposed (Patent Document 7).
In the above-mentioned biological component detection, a single element or an array of elements of InGaAs, PbS, Ge, HgCdTe, an extended-InGaAs including multistage step buffer layers, or the like is used in a spectroscopic device for near-infrared light. A light-receiving wavelength range common to all the above-mentioned biological component detection devices is 1 to 1.8 μm. However, some of the devices determine the upper limit of the light-receiving wavelength range to about 2.0 μm or 2.5 μm.
As described above, as for InGaAs, it is necessary to extend the sensitivity to the long-wavelength side of the near-infrared region. To improve the sensitivity, the methods below have been proposed.
(K1) The indium (In) proportion of an InGaAs absorption layer is increased, and lattice mismatching between the absorption layer and an InP substrate is absorbed by interposing step buffer layers, in which the In proportion is changed stepwise, therebetween (Patent Document 8).(K2) Nitrogen (N) is incorporated in an InGaAs absorption layer to form a GaInNAs absorption layer (Patent Document 9). Lattice matching with an InP substrate is satisfied by incorporating a large amount of N.(K3) An extension of the light-receiving wavelength range to the long-wavelength side is realized by providing a type-II multiple quantum well structure composed of GaAsSb and InGaAs (Non-Patent Document 2). Lattice matching with an InP substrate is satisfied.(K4) Formation of a two-dimensional array is realized by forming element separation trenches between light-receiving elements (pixels) by wet etching (Patent Document 10).Non-Patent Document 1: Masao Nakayama “Technology trend of infrared detectors” Sensor Technology, 1989 March issue (Vol. 9, No. 3), p. 61-64Non-Patent Document 2: R. Sidhu, “A Long-Wavelength Photodiode on InP Using Lattice-Matched GaInAs—GaAsSb Type-II Quantum Wells, IEEE Photonics Technology Letters, Vol. 17, No. 12 (2005), pp. 2715-2717Patent Document 1: Japanese Unexamined Patent Application Publication No. 2002-065645Patent Document 2: Japanese Unexamined Patent Application Publication No. 11-216131Patent Document 3: Japanese Unexamined Patent Application Publication (Translation of PCT Application) No. 2005-519682Patent Document 4: Japanese Unexamined Patent Application Publication No. 11-128209Patent Document 5: Japanese Unexamined Patent Application Publication No. 2001-95806Patent Document 6: Japanese Unexamined Patent Application Publication No. 2005-83901Patent Document 7: Japanese Unexamined Patent Application Publication No. 10-118108Patent Document 8: Japanese Unexamined Patent Application Publication No. 2002-373999Patent Document 9: Japanese Unexamined Patent Application Publication No. 9-219563Patent Document 10: Japanese Unexamined Patent Application Publication No. 2001-144278